Farm tractor operators spend a major part of their life on board of such vehicles, exposed to whole body vibrations potentially harmful for health. The assessment of the vibration level is therefore of crucial importance. This paper deals with the experimental measurement of the vibration acting on the operator body. In particular, the acceleration occurring at the cabin and at the seat are measured. The acceleration components actually relevant for the comfort of the operator are identified. Different farm tractors running on well-defined irregularities able to force at resonance the cabin and the seat have been considered. The six rigid body motions of the cab have been measured by means of accelerometers and gyros. The vertical motion of the seat was recorded. Based on the acquired data, different comfort indices have been computed and compared with the subjective comfort perception of different drivers. It turns out that to correctly assess the vibration behaviour of farm tractors and the comfort of the operators, the measurement of only vertical acceleration is not sufficient and a huge number of measurements are needed.